compounds. Some of these will be discussed in the following section.
Besides human-caused emissions, air quality is affected by environmental factors. A moun-
tain range can trap pollutants on its leeward side. Winds can move pollutants into or out
of a region. Pollutants can become trapped in an air mass as a temperature inversion traps
cool air beneath warm air. If the inversion lasts long enough, pollution can reach dangerous
levels. Pollutants remain over a region until they are transported out of the area by wind,
diluted by air blown in from another region, transformed into other compounds, or carried
to the ground when mixed with rain or snow.
As a result of the Clean Air Act, air in the United States is much cleaner. Visibility is
better and people are no longer incapacitated by industrial smog. Still, in the United States,
industry, power plants and vehicles put 160 million tons of pollutants into the air each year.
Some of this smog is invisible and some contributes to the orange or blue haze that affects
many cities (Figure22.2).
Table22.1lists the smoggiest cities in 2007: six of the 10 are in California. The state has
the right conditions for collecting pollutants including mountain ranges that trap smoggy
air, arid and sometimes windless conditions, and lots and lots of cars.
Figure 22.2: Smog over Los Angeles as viewed from the Hollywood Hills. ( 10 )Table 22.1: Smoggiest Cities, 2007Rank City, State
1 Los Angeles, California
2 Bakersfield, California
3 Visalia-Porterville, California
4 Fresno, California
5 Houston, Texas
6 Merced, California
7 Dallas-Fort Worth, Texas
8 Sacramento, California
9 New York, New York
10 Philadelphia, Pennsylvania